Human cytomegalovirus (HCMV) infections are commonly associated with immunosuppression. A direct effect of this virus on lymphocyte functions in vitro, however, has not been shown. We have been investigating the effects of low-passage, fresh isolates of HCMV (cell-free and cell-associated) on natural killer cell (NK) activity and T cell proliferation. Cell-associated, low-passage isolates of HCMV markedly depressed NK activity. Suppression of NK activity was clearly manifested only after seven days of culture and could not be correlated with titer of virus or cell viability. Addition of interferon-alpha (IFN-alpha) but not interleukin-2 (IL-2) partially reconstituted the response, whereas depletion of infected monocytes prevented inhibition of NK-mediated lysis. The effects of cell-free and cell-associated isolates of HCMV on T cell proliferation differed in several respects from suppression of NK activity. Both cell-associated and cell-free isolates of HCMV completely abrogated antigen-specific and mitogen responses. This effect was apparent after only three days of culture with virus and was not reversed by either IFN or IL-2. Cell-associated strain AD169 also induced suppression but to a lesser extent. From observations reported here and other data, we suggest that HCMV can cause direct suppression of lymphocyte functions.